It is generally known to provide bipolar separators and seals that separate fuel and oxidant gases in SOFC systems.
Fuel cells are well known electrochemical systems that generate electrical current by chemically reacting a fuel gas and an oxidant gas on the surfaces of electrodes. Conventionally, the oxidant gas is oxygen or air, and in high temperature (600° C. to 1000° C.) SOFC the fuel gas can be hydrogen or a mixture of hydrogen, carbon monoxide, and traces of hydrocarbons. The fuel gas may also contain non-fuel gases including nitrogen, water vapor and carbon dioxide. Each cell produces a potential of less than 1 volt, so multiple cells are typically connected in series to produce a higher, more useful voltage. The series interconnection is often accomplished by constructing a bipolar stack of planar cells such that current flows from the anode of one cell to the cathode of the next cell. The stack output current is collected from the top and bottom cells at a voltage equal to the sum of the voltages of the individual cells. Fuel gas and oxidant gas must be supplied to each cell in the stack, while being kept separate so that they do not react with each other except on the surfaces of the electrodes. Direct reactions can cause a loss in energy conversion efficiency, and may generate high temperatures that damage the cell or stack structures. Therefore, barrier structures that separate fuel gas from oxidant gas provide an important function in fuel cell stacks. Two types of barriers exemplify these structures: bipolar separator plates (hereinafter also referred to as “bipolar separators”) and seal gaskets.
A bipolar separator connects the anode of each cell in a stack to the cathode of an adjacent cell. These bipolar separators are in contact with the fuel gas on the anode side and the oxidant gas on the cathode side, and must be largely impermeable to these gases. In addition, they must be electronic conductors able to carry the current from one cell to the next. Further, they must be ionic non-conductors to avoid unwanted reactions between the fuel and oxidant gases. Finally, they must not deteriorate from interactions with the fuel and oxidant gases at elevated operating temperatures, and must have thermal expansion characteristics compatible with adjacent cells.
A number of metals and alloys have been investigated for possible use as separator plates. In general, pure metals and alloys that resist oxidation damage do so by forming an adherent oxide layer that is a barrier to further oxygen attack. While this oxide layer protects the bulk metal, the oxides are generally electronic insulators and severely restrict current flow. Chromium alloys such as high chromium ferric steel are an exception, and form an electronically conductive, adherent oxide. An example is iron with 18% chromium and 1% aluminum. One problem with such alloys is that the chromium forms volatile compounds in an oxidizing environment at the operating temperatures. These compounds tend to migrate and degrade other cell components, particularly at the cathode-electrolyte interface, as described in U.S. Pat. No. 6,444,340 (Jaffrey) and U.S. Pat. No. 5,942,349 (Badwal et al.). Jaffrey teaches that chromium can be replaced with noble metal conductors between the cathode and anode sides of a nonconductive bipolar separator to form the electrical interconnection. U.S. Pat. No. 6,183,897 (Hartvigsen et al.) follows a similar approach. In Badwal et al., a coating is applied to the cathode side of a chromium-containing bipolar separator, thereby capturing and separating the chromium-containing vapor. U.S. Pat. No. 6,280,868 (Badwal et al.) addresses nickel and chromium interdiffusion and oxidation problems on the anode side of a chromium-containing bipolar separator, and applies one or more noble metal layers as a protective barrier. For at least the reasons discussed above, chromium-based alloys are not preferred materials for use in bipolar separators.
Doped lanthanum chromite provides a nonmetallic alternative to chromium-based alloys, where doped lanthanum chromite is an electronically conductive, ionically non-conductive relatively impermeable ceramic. Moreover, doped lanthanum chromite is compatible with common fuel and oxidant gases, does not evolve chromium vapors, and has favorable expansion properties. It has been used successfully as a bipolar separator in the form of self-supporting separator plates made from bulk material and as thin films applied to cathode surfaces. U.S. Pat. No. 5,958,304 (Khandkar et al.) provides an example of formulations and processes for making self-supporting doped lanthanum-chromite separator plates. Problems with such separator plates include their high cost, and excessive weight and volume. Thin (30 to 100 micron) doped lanthanum chromite films applied to the cathode are a potential improvement, as described in U.S. Pat. No. 5,391,440 (Kuo et al.). Such films can be applied by electrochemical vapor deposition (EVD) and plasma spray with high temperature heat treatment to reduce porosity, but undesirably require processing steps at between 1350° C. and 1450° C. that are time-consuming and expensive. These high firing temperatures may damage other components, limiting their use in fabrication approaches where multiple cell components are combined green and co-fired. Further, the range of compositions that can be applied by EVD are limited, resulting in non-optimum thermal expansion and conductivity.
Seal gaskets are similar to bipolar separators in that they also form barriers between fuel and oxidant gases. Flow is blocked between internal openings and the exterior edge of a gasket, and from one internal opening to another. Some seal surfaces contact fuel gas, and other surfaces contact oxidant gas, resulting in requirements similar to bipolar separators. The seal gaskets must be ionic non-conductors, and largely impermeable to fuel and oxidant gases. Further, they must not deteriorate from interactions with the fuel and oxidant gases at elevated operating temperatures, and must have thermal expansion characteristics compatible with adjacent cells. Seal gaskets differ from bipolar separators in that seal gaskets are not required to be electronic conductors.
Glass-based seal gaskets are described in U.S. Pat. No. 5,453,331 (Bloom et al.) and U.S. Pat. No. 6,271,158 (Xue et al.). A glass and filler are selected such that the seal is somewhat viscous and compliant at the cell operating temperature, and thereby adjusts to fill the gaps. One problem is that the seals transition to elastic solids as the cell and stack assembly cools. This may generate significant stresses unless the solids are a good thermal expansion match with the cell and stack components. Another problem is that glasses often wet the cell and stack materials, and therefore migrate from their original locations. A further problem is that the glasses tend to interdiffuse with the cell materials, changing the properties of both substances.
U.S. Pat. No. 6,106,967 (Virkar et al.) addresses the problems of glass seals by employing a thin metallic foil as a combined bipolar separator and sealing gasket. The foil is sufficiently compliant in compression to conform to the mating surfaces and provide a seal. Further, the foil is thin enough such that it does not generate excessive thermal stresses, even with some mismatch in thermal expansion characteristics. Virkar et al. indicates that the foil should be a superalloy containing chromium, which leads to the difficulties with chromium discussed above.
The above-described bipolar separator plates and seals are not made of suitable materials to ensure durable electrical conductivity for use in SOFC cell power generation systems.